HONOLULU – Federal officials are investigating why two passenger jetliners got too close to each other in the air near Hawaii.

They say a United Airlines pilot quickly descended after getting a cockpit alert about an oncoming US Airways flight on April 25.

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A government official familiar with the investigation says the planes were about 8 miles apart horizontally when the United plane maneuvered to avoid the other plane. He says he doesn't know the planes' vertical distance or whether they would have collided had they stayed on course.

The official says the planes were 5.3 miles apart horizontally and 800 feet apart vertically about 12 seconds later.

The official insisted on anonymity because he was not authorized to release the preliminary information.

National Transportation Safety Board spokesman Terry Williams says investigators are reviewing air traffic control procedures, and audio and radar data to determine what happened.